1. Field of the Invention
This invention relates to a hydraulic drive system for a construction machine which, like a hydraulic excavator having a first boom, a second boom and an arm, is provided with at least three elongated members and permits mounting of an attachment such as vibrator or gripper on a free end portion of the most distal elongated members.
2. Description of the Related Art
FIG. 7 is a hydraulic circuit diagram showing a conventional hydraulic drive system of the above-mentioned type for a construction machine.. The conventional art shown in FIG. 7 is applied, for example, to a hydraulic excavator. The conventional art will hereinafter be described with reference to FIG. 7.
The hydraulic excavator depicted in FIG. 7 is provided with elongated members, for example, three elongated members although they are not shown there. The first elongated member is a first boom connected to a pivot cab turnably in a vertical plane, the second elongated member is a second boom connected to the first boom turnably in the vertical plane, and the third elongated member is an arm connected to the second boom turnably in the vertical plane. To conduct usual work such as digging, a bucket is mounted on a free end of the arm. Upon conducting work such as breaking, demolition or the like of rocks or a building, an attachment such as a vibrator or gripper is mounted on the free end of the arm in place of the bucket.
As is illustrated in FIG. 7, the hydraulic excavator is provided, for example, with a first pump 13 having a displacement-varying mechanism 13a, a second pump 14 having a displacement-varying mechanism 14a, and a reservoir 22 with fluid stored therein for suction by these pumps 13,14.
The hydraulic excavator is also provided with a first hydraulic cylinder for turning the above-mentioned first elongated member, namely, a first boom cylinder 1 for turning an unillustrated first boom; a second hydraulic cylinder for turning the second elongated member, namely, a second boom cylinder for turning an unillustrated second boom; a third hydraulic cylinder for turning the third elongated member, namely, an arm cylinder 3 for turning an unillustrated arm; a bucket cylinder 4 for turning an unillustrated bucket; and an actuator 5 for turning the above-mentioned attachment (not shown).
A group of flow control valves connected to the first pump 13 includes, for example, a flow control valve 11 for the bucket, said flow control valve 11 having a center bypass passage and being capable of controlling a flow of pressure fluid delivered from the first pump 13 to feed it to the bucket cylinder 4; a primary flow control valve for the first elongated member, namely, a primary flow control valve 6 for the first boom, said flow control valve 6 being connected to the first pump 13 in parallel with the flow control valve 11 for the bucket, having a center bypass passage and being capable of controlling a flow of pressure fluid delivered from the first pump 13 to feed it to the first boom cylinder 1; and a primary flow control valve for the third elongated member, namely, a primary flow control valve 9 for the arm, said flow control valve 9 having a center bypass passage, being connected in tandem to a downstream side of the primary flow control valve 6 for the fist boom and being capable of controlling a flow of pressure fluid delivered from the first pump 13 to feed it to the arm cylinder 3.
A group of flow control valves connected to the second pump 14 includes, for example, an additional flow control valve for the third elongated member, namely, an additional flow control valve 10 for the arm, which can control a flow of pressure fluid delivered from the second pump 14 to feed it to the arm cylinder 3; an additional flow control valve for the first elongated member, namely, an additional flow control valve 7 for the first boom, which can control a flow of pressure fluid delivered from the second pump 14 to feed it to the first boom cylinder 1; and a reserve flow control valve 12 which can control a flow of pressure fluid delivered from the second pump 14 to selectively feed it to one of the above-mentioned second boom cylinder 2 and the above-mentioned attachment-driving actuator 5. The additional flow control valve 10 for the arm, the additional flow control valve 7 for the first boom and the reserve flow control valve 12 are connected to the second pump 14 in parallel with each other.
The primary flow control valve 9 for the arm and the additional flow control valve 10 for the arm are changed over by an arm operating device, for example, by a pilot valve 15 for the arm, which generates a pilot pressure. The reserve flow control valve 12 is changed over by a second boom/attachment operating device, for example, by a pilot valve 16a for the second boom and attachment, which generates a pilot pressure. The primary flow control valve 6 for the first boom and the additional flow control valve 7 for the first boom are changed over by a first boom operating device, for example, by a pilot valve 17 for the first boom, which generates a pilot pressure. The flow control valve 11 for the bucket is changed over by a bucket operating device, for example, by a pilot valve 19 for the bucket, which generates a pilot pressure.
The reserve flow control valve 12 is communicated to a directional control valve 52, to which the second boom cylinder 2 and the attachment-driving actuator 5 are connected. When the directional control valve 52 is maintained, for example, in a lower position as shown in FIG. 7, the reserve flow control valve 12 an the second boom cylinder 2 are in communication with each other while the reserve flow control valve 12 and the actuator 5 are cut off from each other. When the directional control valve 52 is changed over into an upper position, the reserve flow control valve 12 and the actuator 5 are brought into communication with each other while the reserve flow control valve 12 and the second boom cylinder 2 are cut off from each other. A control compartment of the directional control valve 52 is designed to be brought into selective communication with one of a hydraulic pressure source 51 and the reservoir 22 via a directional control valve 50.
With the conventional art constructed as described above, operations are performed as will be described next by way of example.
[single operation of the second boom]
When the directional control valve 50 is maintained in a right position as shown in FIG. 7, the control compartment of the directional control valve 52 is in communication with the reservoir 22 so that the directional control valve 52 is maintained in the lower position as shown in FIG. 7. Therefore, the reserve flow control valve 12 and the second boom cylinder 2 are in communication with each other via the directional control valve 52.
When the pilot valve 16a is operated in this state, the reserve flow control valve 12 is changed over, and pressure fluid is fed from the second pump 14 to the second boom cylinder 2 via the reserve flow control valve 12 and the directional control valve 52. The second boom cylinder 2 is hence operated, resulting in single operation of the unillustrated second boom.
[single operation of the attachment]
When the directional control valve 50 is changed over from the its position shown in FIG. 7 into a left position in FIG. 7, the control compartment of the directional control valve 52 and the hydraulic pressure source 51 are brought into communication with each other via the directional control valve 50, and the pressure fluid delivered from the hydraulic pressure source 51 is fed to the control compartment of the directional control valve 52. As a result, the directional control valve 52 is changed over into the upper position shown in FIG. 7, and the reserve flow control valve 12 and the actuator 5 are brought into communication with each other via the directional control valve 52.
When the pilot valve 16a is operated in this state, the reserve flow control valve 12 is changed over so that pressure fluid is fed from the second pump 14 to the actuator 5 via the reserve flow control valve 12 and the directional control valve 52. The actuator 5 is hence driven, resulting in single operation of the unillustrated attachment.
[Combined operation of the first boom, the second boom and the arm]
When the pilot valve 15 for the arm, the pilot valve 17 for the first boom and the pilot valve 16a are operated in the state that the directional control valves 50,52 are maintained in their respective positions shown in FIG. 7 and the reserve directional control valve 12 and the second boom cylinder 2 are in communication with each other as shown in FIG. 7, the primary flow control valve 9 for the arm, the additional flow control valve 10 for the arm, the primary flow control valve 6 for the first boom, the additional flow control valve 7 for the first boom and the reserve flow control valve 12 are changed over. As a result, for example, pressure fluid is fed from the first pump 13 to the first boom cylinder 1 primarily via the primary flow control valve 6 for the first boom so that the first boom cylinder 1 is actuated, and on the other hand, pressure oil is fed from the second pump 14 to the arm cylinder 3 primarily via the primary flow control valve 10 for the arm and also to second boom cylinder 2 primarily via the reserve flow control valve 12, whereby the arm cylinder 3 and the second boom cylinder 2 are actuated. By these actuations, combined operation of the unillustrated first boom, second boom and arm is performed.
[Combined operation of the first boom, the arm and the attachment]
When the pilot valve 15 for the arm, the pilot valve 17 for the first boom and the pilot valve 16a are operated in the state that the directional control valve 50 and the directional control valve 52 have been changed over to the left position and the upper position, respectively, from their respective positions shown in FIG. 7 and the reserve flow control valve 12 and the actuator 5 have been brought into communication with each other, the primary flow control valve 9 for the arm, the additional flow control valve 10 for the arm, the primary flow control valve 6 for the first boom, the primary flow control valve 7 for the first boom and the reserve flow control valve 12 are changed over to actuate or drive the first boom cylinder 1, the arm cylinder 3 and the actuator 5 in a similar manner as described above. Combined operation of the unillustrated first boom, arm and attachment is therefore performed.
The above-described conventional art permits combined operation of the first boom, the second boom and the arm or combined operation of the first boom, the arm and the attachment. It however cannot perform combined operation which includes operation of the second boom and the attachment. For example, it is impossible to operate the arm, the second boom an the attachment in combination.
In work, such as breaking or demolition, which is conducted by using an attachment, the operation-feasible range therefore tended to be limited. To change an operation-feasible range already set beforehand, complicated valve operation may be needed. For example, the directional control valve 50 is once changed over into the right position in FIG. 7 to change over the directional control valve 52 into the lower position shown in FIG. 7. As a consequence, the second boom cylinder 2 is caused to extend. Then, the directional control valve 50 is changed over into the left position in FIG. 7 to change over the directional control valve 52 into the upper position depicted in FIG. 7. The actuator 5 is hence driven to perform work by the attachment. Similarly, the directional control valve 50 is once changed over into the right position in FIG. 7 to change over the directional control valve 52 into the lower position shown in FIG. 7. As a consequence, the second boom cylinder 2 is caused to contract. Then, the directional control valve 50 is changed over into the left position in FIG. 7 to change over the directional control valve 52 into the upper position depicted in FIG. 7. The actuator 5 is hence driven to perform work by the attachment. This has led to the problem that no improvement can be expected in the efficiency of work by the attachment.
Further, due to the need for frequent change-over operation of the directional control valve 50, the operator tends to feel irksome and tired, resulting in the problem that the accuracy of work tends to drop.
Concerning the conventional art described above, the hydraulic excavator was referred to as a construction machine, and the problems associated with combined operation of the second boom and the attachment were described. In the case of a hydraulic drive system which is suited for use with a construction machine having three or more elongated members and an attachment and is of such construction as selectively driving the attachment and one of the elongated members, problems arise likewise from the combined operation of the attachment and the particular elongated member. A technique similar to the above-described conventional art is disclosed, for example, in JU kokai 2-15650.